This invention relates to the separation of the isotopes of hydrogen from a mixture thereof, and is directed particularly to the separation of deuterium from a gaseous mixture comprising essentially protium, hereinafter called "hydrogen" and deuterium and the production of heavy water with the deuterium.
The advent of atomic energy processes on a commercial scale has created, and will in the future continue to create, large demands for relatively pure single isotopes. Many methods have been proposed and employed in the prior art for the separation of isotopic mixtures and/or enrichment thereof. Such processes include fractional distillation, gaseous diffusion, electromagnetic methods, mass spectrographic methods, chemical isotopic exchange reactions, selective electrolysis, and the like. These prior art processes are generally very expensive in commercial operations and require large and expensive outlays of equipment. The separation factors for most of the existing prior art processes are very low, even for the hydrogen isotopes and a great number of stages must be employed to achieve significant separations. Furthermore, the separation or enrichment of isotopes by the foregoing methods usually involves a large hold-up of materials within the process.
Heavy water is water in which hydrogen of mass number one is replaced by its isotope deuterium of mass number two. Heavy water is employed as a moderator in nuclear reactors using slightly enriched uranium as a fuel. It is also used as a combined moderator-coolant in reactors using natural uranium as a fuel. The function of a moderator in a reactor is to slow down the neutrons so as to increase the probability that they will be absorbed by the uranium atoms thus causing them to fission. The so-called CANDU (Canadian Deuterium Uranium) reactor is an example of those using heavy water and natural uranium.
Deuterium occurs in natural water, hydrogen, methane, etc. in a ratio of about 100-150 parts per million (D/H molecular ratio). The ratio in hydrogen varies somewhat with the source of the hydrogen. Because deuterium is an isotope of hydrogen, the two cannot be separated by ordinary chemical methods. There are, however, a number of prior art processes by which deuterium in the form of heavy water can be separated from hydrogen or from ordinary or natural water. These include distillation, electrolysis, diffusion, normal kinetic isotope effect and various chemical exchange processes. All of these are expensive, requiring either large capital expenditures or large amounts of energy, or both.
Of the many processes available, the process used most to produce heavy water is a dual temperature chemical exchange process, the so-called GS or sulfide process. In this process, natural water is concentrated to about 10% or more heavy water by chemical exchange utilizing hydrogen sulfide. The water is then concentrated further by suitable well-known means such as vacuum distillation to yield a reactor grade product (approximately .gtoreq. 99.8 mol percent heavy water).
The GS process takes advantage of the fact that in the process or reaction ##STR1## and H.sub.2 S, deuterium tends to concentrate in the liquid component at the lower temperature and in the gas at the higher temperature. By causing the reaction to go first in one direction and then in the other and drawing off the enriched gas and liquid streams at appropriate points and passing them on to further steps, natural water is enriched to the point that it becomes economic to concentrate it further by other means. It is by this process that--excluding the production of China and the USSR--approximately 90% of the present world supply of heavy water is produced.
Although the advantages of the sulfide process have caused it to be selected and preferred over all other prior art processes, there is a major disadvantage of the sulfide process in that large quantities of a toxic gas are used. Further, since it is used, not only the piping and vessels must be fabricated to at least substantially the same standards as apply to nuclear power plants, but also a large exclusionary area around the plant is necessary.